Peroxisome Proliferator Activated Receptors (PPARs) are members of the nuclear hormone receptor superfamily, a large and diverse group of proteins that mediate ligand-dependent transcriptional activation and repression. Three subtypes of PPARs have been isolated: PPARα, PPARγ and PPARδ.
The expression profile of each isoform differs significantly from the others, whereby PPARα is expressed primarily, but not exclusively in liver; PPARγ is expressed primarily in adipose tissue; and PPARδ is expressed ubiquitously. Studies of the individual PPAR isoforms and ligands have revealed their regulation of processes involved in insulin resistance and diabetes, as well as lipid disorders, such as hyperlipidemia and dyslipidemia. PPARγ agonists, such as pioglitazone, can be useful in the treatment of non-insulin dependent diabetes mellitus. Such PPARγ agonists are associated with insulin sensitization.
PPARα agonists, such as fenofibrate, can be useful in the treatment of hyperlipidemia. Although clinical evidence is not available to reveal the utility of PPARδ agonists in humans, several preclinical studies suggest that PPARδ agonists can be useful in the treatment of diabetes and lipid disorders.
The prevalence of the conditions that comprise Metabolic Syndrome (obesity, insulin resistance, hyperlipidemia, hypertension and atherosclerosis) continues to increase. New pharmaceutical agents are needed to address the unmet clinical needs of patients.
PPARδ agonists have been suggested as a potential treatment for use in regulating many of the parameters associated with Metabolic Syndrome and Atherosclerosis. For example, in obese, non-diabetic rhesus monkeys, a PPARδ agonist reduced circulating triglycerides and LDL, decreased basal insulin levels and increased HDL (Oliver, W. R., et al.; Proc Natl Acad Sci 98:5306-5311; 2001). The insulin sensitization observed with the use of a PPARδ agonist is thought to be in part due to decreased myocellular lipids (Dressel, U., et al.; Mol Endocrinol 17:2477-2493; 2003).
Further, atherosclerosis is considered to be a disease consequence of dyslipidemia and may be associated with inflammatory disease. C-reactive protein (CRP) production is part of the acute-phase response to most forms of inflammation, infection and tissue damage. It is measured diagnostically as a marker of low-grade inflammation. Plasma CRP levels of greater than 3 mg/L have been considered predictive of high risk for coronary artery disease (J. Clin. Invest 111: 1085-1812; 2003).
PPARδ agonists are believed to mediate anti-inflammatory effects. Indeed, treatment of LPS-stimulated macrophages with a PPARδ agonist has been observed to reduce the expression of iNOS, IL12, and IL-6 (Welch, J. S., et al.; Proc Natl Acad Sci 100:6712-67172003).